I use http://www.xml-sitemaps.com/ It's free and easy enough. Make sure you use an ftp program to upload, my sitemaps never work when I upload with dreamweaver.
Google has a stack of online generators listed near the bottom of http://code.google.com/sm_thirdparty.html
I tried one online sitemap generator to run against my site My site is an web application. It has only about 40 pages wich has generated content again and again. For example I have over 100 users. Each user has profile (page myprofile.aspx). The page for profile is common for all users. There is user list on other page. Sitemap generator went over 100 time to myprofile.aspx. Then there is a link 'Send Message' on myprofile page. This link references to newpage.aspx page. Sitemap generator went on this page over 100 time again and so on. My point is if I web-application I must create sitemap file by my own to skip unneeded pages.
if you have more than 500 pages, online sitemap makers are not good . have you tried to use gsitecrawler? it's great and free sitemap maker
Or if you have only 5 pages that will appear 100 time to be slightly different for every unique user. 5 * 100 = 500 items in the autogenerated sitemap.
In theory In reality I believer Google sitemaps "audit" your site - this may result in an increased rank in the medium term but for now, I think it's purely for Google to inspect sites in detail. I've seen no evidence whatsoever of it helping with indexing.